A casting magnesium alloy for providing improved thermal conductivity

ABSTRACT

A casting magnesium alloy for providing improved thermal conductivity A magnesium alloy for providing improved thermal conductivity includes from 1 wt.% to 5 wt.% of lanthanum, from 1 wt.% to 5 wt.% of cerium or a combination thereof, and from 0.5 wt.% to 3 wt.% of neodymium, from 0.5 wt.% to 3 wt.% of gadolinium or a combination thereof, and from 0.0 wt.% to 0.2 wt.% of yttrium, and up to 0.8 wt.% of praseodymium, and up to 0.8 wt.% manganese, and up to 1.0 wt.% aluminium, and up to 0.8 wt.% zinc, and up to 20ppm beryllium, and with balanced magnesium and inevitable impurities.

A Casting Magnesium Alloy For Providing Improved Thermal Conductivity

The invention relates to a casting magnesium alloy, in particular the die-casting magnesium alloy that can provide the significantly improved thermal conductivity in comparison with the commonly used AZ91D (Mg-9wt.%Al-1wt.%Zn) die-casting magnesium alloy. The typical thermal conductivity of the die-casting magnesium alloys of the present invention is at a level of more than110 W/m.K (that is, Watts per metre-Kelvin) under as-cast condition and room temperature (about 20° C.), which is close to the thermal conductivity of the commonly used A380 (Al-9wt.%Si-3wt.%Cu) die-casting aluminium alloy.

In competing with aluminium alloys as structural materials, magnesium alloys have advantages of low density, high strength ratio, and better electromagnetic shielding properties and castability. These result in the preferential applications of magnesium alloys in the components where the lightweight and electromagnetic shielding is critical.

However, the currently used magnesium alloys have inferior thermal conductivity in comparison with its competitive materials of aluminium alloys. For examples, the thermal conductivity of pure magnesium is 156 W/m·K at 25° C., but that of pure aluminium is 205

W/m·K at 25° C. Similarly, the most commonly used die-casting alloys show significant differences in the thermal conductivity. The commonly used AZ91D die-casting magnesium alloy has the thermal conductivity of 51 W/m·K at 20° C., and the commonly used A380 die-casting aluminium alloy has the thermal conductivity of 108 W/m·K at 20° C. Therefore, the ratio of thermal conductivity is 0.76 between pure Mg and Al but is 0.47 between AZ91D and

A380.

Although the thermal conductivity of a component having the same weight is similar for aluminium and magnesium, the thermal conductivity for AZ91D magnesium components is about half that of comparable A380 aluminium components. Therefore, it is desirable to produce magnesium alloys having improved thermal conductivity.

High thermal conductivity is desirable for increasing the capacity of heat exchange and heat sink of engineering components. Improved thermal conductivity can reduce the working temperatures of final products and therefore increase the energy efficiency and product life.

This is particularly important when combined with the advantages of magnesium alloys, namely their low density, electromagnetic shielding properties and castability.

In general, the addition of solute elements into pure magnesium reduces the thermal conductivity, although alloying is in many instances essential in order that the pure metals can be useful in engineering. Therefore, it is a favoured method to use alloying elements to improve the thermal conductivity of magnesium alloys. Prior attempts to provide improved thermal conductivity of magnesium alloys using different alloying elements at different levels have been disclosed and a number of these disclosures are set out in Table 1 below, in which the alloys include casting alloys and wrought alloys.

The technical routes of the prior arts can be described as (1) using the currently available magnesium alloys as thermal conductive alloys, which include Mg-Si-Zn based alloys; (2) modifying the existing magnesium alloys with special elements at low levels, which include alkaline earth metals and rare earth (Re) elements; (3) developing new alloys, such Mg-Sn- Zn-Zr based alloys.

However, the existing research in the field of high thermal conductive casting magnesium alloys has the following problems: (1) in casting alloys, the magnesium alloys contain high solute contents cannot provide sufficient thermal conductivity such as the commonly used AZ91D alloy, more particularly, are not capable of competing with its aluminium alloy counterparts; (2) in wrought alloys, low level solutes are used. Therefore, the alloy can provide better thermal conductivity than casting alloys. However, the current market share of wrought magnesium alloys is less than 5%; (3) the contradiction between the thermal conductivity and castability of the alloy is a serious problem for the development of thermal conductive magnesium alloys. The castability is often not sufficiently good for the new magnesium alloys that have better thermal conductivity in comparison with the popular

AZ91D alloy; (4) precious metals such as Ag are used, thus increasing the materials cost, limiting the material source and making the alloys difficult to popularize and put into civil use; (5) heat treatment of the alloy is usually employed, which increases the energy consumption during manufacturing.

In addition, it has been discovered that many of the prior art alloys shown in Table 1 have at least some of the following disadvantages:

⊇ they are not suitable for high pressure diecasting (HPDC) because the alloy is more likely to stick to the mould

⊇ also HPDC results in defects in the alloy

⊇ they are unlikely to have a good thermal conductivity at higher temperatures that ambient (for example over 150° C.)

U.S. Ser. No. 2010/0310409 A1 (Gibson et al.) discloses a magnesium based alloy consisting of, by weight: 2-5% rare earth elements, wherein the alloy contains lanthanum and cerium as rare earth elements and the lanthanum content is greater than the cerium content; 0.2-0.8% zinc;

0-0.15% aluminium; 0-0.5% yttrium or gadolinium; 0-0.2% zirconium, 0-0.3% manganese;

0-0.1% calcium; 0-25 ppm beryllium; and the remainder being magnesium except for incidental impurities.

US 2009/0136380 A1 (Gibson et al.) discloses a magnesium-based alloy consists of 1.5-4.0% by weight rare earth element(s), 0.3-0.8% by weight zinc, 0.02-0.1% by weight aluminium, and 4-25 ppm beryllium. The alloy optionally contains up to 0.2% by weight zirconium, 0.3% by weight manganese, 0.5% by weight yttrium and 0.1% by weight calcium. The remainder of the alloy is magnesium except for incidental impurities.

US 2008/0193322 Al (Gibson et al.) discloses a magnesium-rare earth-yttrium-zinc alloy consists of 0.2-1.5% by weight zinc and rare earth(s) (RE) and yttrium in amounts which fall within a quadrangle defined by lines AB, BC, CD and DA wherein: A is 1.8% RE-0.05% Y, B is 1.0% RE-0.05% Y, C is 0.2% RE-0.8% Y, and D is 1.8% RE-0.8% Y.

In accordance with a first aspect of the invention, there is provided a magnesium alloy including:

-   -   a. from 1 wt.% to 5 wt.% of lanthanum, from 1 wt.% to 5 wt.% of         cerium or a combination thereof, and     -   b. from 0.5 wt.% to 3 wt.% of Neodymium (preferably from 0.5         wt.% to 2 wt.%), from 0.5 wt.% to 3 wt.% (preferably 0.5 wt.% to         1.5 wt.%) of Gadolinium or a combination thereof, and

c. up to 0.2 wt.% of Yttrium, and

d. up to 0.8 wt.% of Praseodymium, and

e. up to 0.8 wt.% Manganese, and

f. up to 1.0 wt.% (preferably up to 0.5 wt%) Aluminium, and

g. up to 0.8 wt.% Zinc, and

h. up to 20ppm beryllium, and

i. with the balance being magnesium and inevitable impurities.

By “a combination thereof” is meant both of the previously listed elements in the amounts listed.

The present invention intends to provide a casting magnesium alloy for significantly improved thermal conductivity in comparison with the commonly used commercial AZ91D alloy. In particular, the claimed alloy can provide the thermal conductivity at a level that A380 Aluminium alloy can. Moreover, the claimed alloy provides good castability and works well under as-cast condition with excellent mechanical properties. In a preferred embodiment of the present invention, there is provided a magnesium alloy for providing improved thermal conductivity more than 110 W/m.K including:

a. from 1 wt.% to 5 wt.% of a first lanthanide (preferably lanthanum), from 1 wt.% to 5 wt.% of a second lanthanide (preferably cerium) or a combination thereof, wherein the first and second lanthanide is the same of different, and

b. from 0.5 wt.% to 3 wt.% of Neodymium, from 0.5 wt.% to 3 wt.% of Gadolinium or a combination thereof, and

c. up to 0.2 wt.% of Yttrium, and

d. up to 0.8 wt.% of Praseodymium, and

e. up to 0.8 wt.% Manganese (with the amount of manganese being non-zero), and

f. up to 1.0 wt.% Aluminium (with the amount of aluminium being non-zero), and

g. up to 0.8 wt.% Zinc (with the amount of zinc being non-zero), and

h. up to 20ppm beryllium, and

i. with the balance being magnesium and inevitable impurities.

Preferably, the magnesium alloy contains:

⋅ one or more elements that is more reactive with aluminium than magnesium in the alloy melt during solidification and therefore forming eutectic phase without destroy the continuity of α-Mg phase in the matrix to reduce the solute concentration in the primary α-Mg phase

⋅ an appropriate amount of manganese to neutralise impurities and improve die releasing capability

⋅ and a small amount of beryllium and/or other elements to protect the oxidation during casting operation

In a preferred embodiment, there is provided a casting magnesium alloy material for providing significantly improved thermal conductivity under as-cast condition comprising at least two elements from rare earth elements, each is at a level of more than lwt.% and less than 5wt.%, one of which is used to provide the improvement of castability and the other one is used to provide strengthening of the alloy. The alloy is capable of providing a thermal conductivity at a level of more than 110W/ m·K under as-cast condition and room temperature (20° C.). Without wishing to be constrained by theory, it is believed that the improved thermal conductivity available with the invention, particularly in combination with good casting characteristics results from the combination of Mg with Re within the given ranges because Re in the alloy can reduce the liquidus temperature and improve the castability. Accordingly, the method used in this patent is that to use Re solute elements to improve the castability. In the meantime, some of the Re elements have high solubility in Mg matrix and capable of providing materials strengthening.

Although the rare earth elements can be any in the Lanthanoids, it is preferred to have La and Ce or any other low-cost rare-earth elements. Therefore, the elements such as Nd and Gd should be added in a controlled amount in the alloy. More importantly, La or Ce can be obtained from recycled materials, which is an effective way to reduce the alloy cost. The elements used in the present invention can be replaced by other elements that promote the formation of intermetallic phases easier than Mg-Al phase in the Mg matrix.

Another element in the alloy is preferably required to have a high solubility in the primary α-Mg phase at elevated temperatures close to its eutectic solidification but has very low solubility at ambient temperature. The element can improve the castability and mechanical properties of the magnesium alloy, and it promotes the formation of precipitates in the primary α-Mg phase after solidification, which can provide effective strengthening in the alloy and reduce the solute concentration in the primary α-Mg phase. More importantly, this can reduce the solute content in the primary α-Mg phase to improve the thermal conductivity.

The maximum solid solubility for alloying elements in magnesium have been reported (at.%) as Ag (3.8), A1(11.6), Ca(0.8), Ce (0.1), Cu (0.15), Gd (4.5), In (19.4), La (0.1), Li (18), Mn (1.0), Nd (0.1), Si (0.003), Sn(3.35), Sr (0), Tb (4.6), Th (0.52), Y (3.35), Zn (2.4), Zr (1.0), Zn(2.4), Tm (6.3), Sc (15), Pb(7.75), Sn (0.35), Yb (1.2), Bi (1.1), Ca (0.82), Ti (0.1) and Au (0.1). Therefore, the options are apparent for the element that has very low solubility at ambient temperature. The elements including Ca, Ce, Cu, La, Nd, Sr, Th, Zr, Sn, Yb, Bi, Ca, Ti and Au can be the candidates. Clearly, the rare earth elements are one of the preferred options because of their low solubility in primary magnesium phase and the multiple functions to promote the formation of eutectic phase. The other elements including Ca, Cu,

Sr, Zr, Sn, Bi, Ti and Au are good options for the alloy. All these elements should be controlled within the maximum solubility at the eutectic reaction temperature.

Manganese (Mn) is an important alloying element for all embodiments of the casting magnesium alloy according to the present invention. The role of Mn includes the neutralisation of Fe to form compact Fe-rich intermetallics, and to promote die releasing during die-casting, which allows the alloy to be die-castable for massive production. For these reasons, Mn content is lower in the castings made by sand casting and other method without using a metallic mould, but higher in the casting methods using metallic mould. The Mn level is in the range of 0.1 to 0.8 wt.%. A more preferred Mn level is in the range of 0.1 to 0.3%.

Beryllium (Be) is commonly added to casting magnesium alloys to prevent oxidation of the magnesium alloy. As little as up to 20ppm causes a protective beryllium oxide film to form on the surface. Preferably, as usual, the Be level is controlled to be about 20ppm.

The present invention also includes products made from the casting magnesium alloys set out a variety of casting operations, which including the common casting methods including but not limited to sand casting, investment casting, lost foam casting, gravity and permanent mould die casting, low pressure die casting, high pressure die casting, and squeeze casting. By the present invention, the alloying elements are selected to promote the alloy not only having improved thermal conductivity, but also having good castability. Therefore, it is suitable for common castings methods, in particular in die-casting operations without soldering problems. This is a huge advantage because die-casting is dominant process for magnesium alloys. In the meantime, die-casting can make thin wall components. Therefore, the alloy is particularly suited for manufacturing products having requirements where the high conductivity and light-weighting are desirable.

In an embodiment of the casting magnesium alloy according to the invention the following levels for the alloying element are selected (all composition percentages are by weight): La 1.0-5.0 wt.%, Ce 1.0-5.0 wt.%, Nd 0.5-2wt.%, Gd 0.5-1.5wt.%, Y 0-0.2wt.%, Pr <0.6wt.%, Mn 0.1-0.4 wt.%, and Al<1.0wt.%, Zn<0.8wt.%, with Be 20ppm and the balanced magnesium and inevitable impurities.

In another embodiment of the casting aluminium alloy according to the present invention of the preferred levels of the alloying element are selected: La 1.0-3.0 wt.%, Ce 1.0-3.0 wt.%, Nd 0.5-1.5wt.%, Gd 0.5-1.5wt.%, Y 0-0.15wt.%, Pr<0.5wt.%, Mn 0.1-0.3 wt.%, and Al<0.6wt.%, Zn<0.5wt.%, with Be12ppm and the balanced magnesium and inevitable impurities.

A number of preferred embodiments of the invention will now be described with reference to the drawings, in which:

FIG. 1 is a graph showing the thermal conductivity as a function of temperature for an alloy in accordance with the invention;

FIG. 2 is a series of photographs of samples of prior art alloys and an alloy in accordance with the invention illustrating high pressure die casting capability; and

FIG. 3 is a graph showing the strain as a function of time for a prior art alloy and an alloy in accordance with the invention.

EXAMPLE 1

Pure magnesium ingots, Mg-5wt.%Mn master alloys and a master alloy containing the mixture of La and Ce in magnesium were used as starting materials. Each element was weighted at a special ratio with an extra amount for burning loss during melting. During alloy making, a top loaded electrical resistant furnace was used to melt the metal in a steel crucible under protection of N₂+(0.05−0.1)vol.% SF₆. A batch of 10 kg alloy was melted at a temperature of 730° C. each time. After the melt was homogenised in the crucible, a mushroom sample with ϕ60×10 mm testing part for composition analysis was made by casting melt directly into a steel mould. The casting was cut off 3mm from the bottom before performing composition analysis. The composition was analysed using an optical mass spectroscopy, in which at least five spark analyses were carried out and the average value was taken as the chemical composition of the alloy. The actual composition of the alloy was Mg-5.5(La, Ce, Nd, Gd, Y)-0.5Al-0.3Zn-0.3Mn (wt.%), with Be also being present at about 20ppm with balanced Mg.

After composition analysis, the casting samples were made by a 4500 kN cold chamber HPDC machine, in which all casting parameters were fully monitored and recorded. The pouring temperature was controlled at 700° C., which was measured by a K-type thermocouple. The die to make standard samples for mechanical properties and thermal conductivity were made under the corresponding optimised condition. The dies were heated by the circulation of mineral oil at 250° C. The samples for thermal conductivity were ϕ12×20 mm round bars. The mechanical properties and thermal conductivity were measured following a standard method defined by ASTM.

A number of other samples were made in accordance with the method of Example 1 using high pressure die casting and gravity and permanent mould die casting. The thermal conductivity was tested under as-cast condition and various temperatures. The thermal conductivity at room temperature is shown in Table 2. The castability was assessed to the commercial A380 and AZ91D alloy and the value in the following Table 2 is a ratio between the filling lengths in permanent mould casting. The thermal conductivity at various elevated temperatures is shown in FIG. 1.

It can be seen that the magnesium alloy samples which are not in accordance with the invention (Samples 2-4) have a lower thermal conductivity than the reference aluminium alloy (Sample 1). However, the magnesium alloy samples which are in accordance with the invention (Samples 5-8) have a comparable thermal conductivity to Sample 1.

TABLE 2 Thermal conductivity Castability (20° C., (ratio Sample Alloy composition (wt. %)* W/m · K) value) 1 Al—9Si—3Cu—1Fe (A380**) 110 100 2 Mg—9Al—1Zn—0.2Mn (AZ91D**)  50  95 3 Mg—4Al—3.5Re—0.25Mn—0.2Zn (AE44**)  82  80 4 Mg—3.5Al—4Re—0.3Mn—0.2Zn (AE44**)  85  85 5 Mg—1.5La—1.1Ce—0.9Nd—1.6Gd—0.1Y—0.2Zn—0.5Al—0.3Mn 110  95 6 Mg—1.1La—1.8Ce—1.2Nd—1.5Gd—0.1Y—0.3Zn—0.6Al—0.2Mn 107  90 7 Mg—1.6La—1.2Ce—1.0Nd—1.0Gd—0.1Y—0.2Zn—0.4Al—0.1Mn 105  90 8 Mg—2.1La—1.3Ce—0.8Nd—1.2Gd—0.1Y—0.2Zn—0.5Al—0.3Mn 108  90 *Be also present in amounts of up to 20 ppm **Commercially available alloys

Turning to FIG. 2, the inventors have tried to test the castability of different alloys with the typical composition disclosed in the art. This is not quantitative method as only relative methods are used in industry. Experiments were carried out to show that the references in Table 1 were not suitable for high pressure die casting and designed for low temperature (<=150° C.) application only, while the alloys of the present invention are suitable for high pressure die casting and could be worked at high temperature (>=200° C.).

The alloys that were tested were those listed in Table 1 above, and these are identified in

FIG. 2 as follows:

(a) CN101113502A, (b) CN101113504A, (c) CN101113503A, (d) CN104046867A, (e) CN102251161A, (f) CN101709418A, (g) CN104152769A, (h) CN102719716A, (i) US20090068053, (j) JP2012197490, (k) CN102560210A, (1) US3094413, (m) the present claimed magnesium alloy with the composition of Mg-1.4La-2.1Ce-0.6Nd-1.2Gd-0.1Y-0.3Zn-0.8Al-0.3Mn.

From FIG. 2, it is clear that many of existing alloys are not able to make sound castings using high pressure die casting process. However, the casting of the alloy (m) in accordance with the invention is sound, which means the castability of the alloy is very good.

Turning to FIG. 3, this shows high temperature creep properties of AZ91D (a typical prior art alloy) and a magnesium alloy in accordance with the invention (Mg-1.4La-2.1Ce-0.6Nd-1.2Gd-0.1Y-0.3Zn-0.8Al-0.3Mn). The short dot curve shows the typical creep properties of the AZ91D alloy at 150° C. and 90 MPa (labelled “reference”). The solid curve shows the typical creep properties of the alloy of the present invention (labelled “present”) at 200 ° C. and 100 MPa.

All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

The disclosures in UK patent application number 1905971.6, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference. Claims 

1. A magnesium alloy including: a. from 1 wt.% to 5 wt.% of lanthanum, from 1 wt.% to 5 wt.% of cerium or a combination thereof, and b. from 0.5 wt.% to 3 wt.% of Neodymium, from 0.5 wt.% to 3 wt.% of Gadolinium or a combination thereof, and c. up to 0.2 wt.% of Yttrium, and d. up to 0.8 wt.% of Praseodymium, and e. up to 0.8 wt.% Manganese, and f. up to 1.0 wt.% Aluminium, and g. up to 0.8 wt.% Zinc, and h. up to 20ppm beryllium, and i. with the balance being magnesium and inevitable impurities.
 2. A magnesium alloy as claimed in claim 1 including: La 1.0-5.0 wt.%, Ce 1.0-5.0 wt.%, Nd 0.5-2wt.%, Gd 0.5-1.5wt.%, Y up to 0.2wt.%, Pr up to 0.6wt.%, Mn 0.1-0.4 wt.%, and Al up to 0.8wt.%, Zn up to 0.8wt.%, with Be 20ppm with the balance being magnesium and inevitable impurities.
 3. A magnesium alloy for providing improved thermal conductivity as claimed in claim 1 or 2 including: La 1.0-3.0 wt.%, Ce 1.0-3.0 wt.%, Nd 0.5-1.5wt.%, Gd 0.5-1.5wt.%, Y up to 0.15wt.%, Pr up to 0.5wt.%, Mn 0.1-0.3 wt.%, and Al up to 0.6wt.%, Zn up to 0.5wt.%, with Be 20ppm with the balance being magnesium and inevitable impurities. 